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  <script>
    //const声明一个只读的常量。一旦声明，常量的值就不能改变。
    const PI = 3.1415;
    //PI = 3;// TypeError: Assignment to constant variable.

    //const一旦声明变量，就必须立即初始化，不能留到以后赋值。
    //const foo;//'const' declarations must be initialized.

    //const的作用域与let命令相同：只在声明所在的块级作用域内有效。
    if (true) {
      const MAX = 5;
    }

    console.log(MAX) // Uncaught ReferenceError: MAX is not defined
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